1. Field of the Invention
The present invention relates to colloidal aqueous rosin dispersions of improved mechanical stability useful in the manufacture of paper. The invention includes the dispersions themselves and processes for the manufacture of the dispersions.
2. Description of the Prior Art
It has long been known that natural rosin can be emulsified in hot dilute aqueous alkali solution and that the product (a colloidal dispersion of rosin in dilute aqueous sodium rosinate solution) is an excellent sizing agent for paper. The best known agent of this type is the "Bewoid" size described in Pulp and Paper by James P. Casey, 2nd Ed., (Vol. 11, p. 1049 ff.).
A disadvantage of these dispersions is that they possess poor mechanical stability in that the dispersed phase aggregates and forms agglomerates when the dispersion is subjected to shear forces (as by passage through a centrifugal or gear pump). Pumps of this type are commonly used in size manufacturing plants and in paper mills, and are quickly fouled and jammed by sticky broken rosin emulsion. Moreover, coagulated rosin particles may find their way onto the paper machine causing picking, breaks, rosin spots, wire filling and other problems.
More recently it has been discovered that the sizing efficiency of rosin is increased when the rosin is reacted with a compound of acidic character containing the --CO--C.dbd.C-- linkage. The product (termed "fortified rosin") has a substantially higher flow point than rosin (usually above 100.degree. C.) and therefore usually cannot be emulsified at atmospheric pressure in the same manner as unfortified size. To avoid the use of autoclave equipment, it is therefore present-day practice to decrease the flow point of the rosin before use by mixing a volatile rosin solvent (typically toluene) into it. The toluene is later recovered from the emulsion by distillation. The process is disclosed in French Pat. No. 781,729 and in U.S. Pat. Nos. 3,565,755 and 3,817,768.
A disadvantage of this process is that the intermediate dispersion (the emulsion of the solvent-softened fortified rosin) is thermally unstable, in that it aggregates when subjected to heat. Aggregation is particularly rapid when the solvent is removed by the efficient steam distillation process.
Aggregation even occurs at room temperature when no solvent or softening agent is present.
Dispersions which have aggregated have no commercial value.
Up to the present, sodium rosinate (or sodium fortified rosinate as the case may be) has been almost exclusively used as the emulsifier for rosin (or for fortified rosin) because it is easily produced by addition of a small amount of sodium hydroxide or carbonate to the aqueous medium used for the emulsification. However, experience has shown that both sodium rosinate and sodium fortified rosinate are unsatifactory in that they do not render colloidal dispersions of rosin or fortified rosin adequately mechanically stable to withstand stringent high-shear elevated temperature conditions.
In the past, a variety of different emulsifying agents has been tried to remedy the situation, but little or no improvement has been achieved.